The present invention relates to a method of manufacturing a continuously-cast cast piece having an inclining composition in which the concentration of a particular solute element is higher in the surface layer of the cast piece than the interior thereof.
Hitherto, various methods of manufacturing a cast piece the component of which is different between the surface layer portion and the interior thereof by continuous casting are proposed.
For example, Japanese Examined Patent Application Publication No. 3-20295 discloses a method of manufacturing a multi-layer cast piece by applying direct current magnetic fluxes to the cast piece in the overall length thereof in a direction perpendicular to a casting direction from a position, which is located below the molten metal level in a continuously-casting mold and is spaced apart therefrom a predetermined distance; and supplying different metal to the upper side and the lower side of a static magnetic field zone that is formed by the direct current magnetic fluxes and acts as a boundary.
Further, Japanese Unexamined Patent Application Publication No. 7-51801 discloses a method of manufacturing a multi-layer steel sheet by pouring molten steel into a continuously-casting mold together with a gas in a vertical direction, reducing the upward flow speed of the molten steel by applying direct current magnetic fluxes to the molten steel in the mold in the overall length thereof from a position above a molten steel pouring position; adding an element different from the components of the molten steel to the molten steel located above the position from which the direct current magnetic field is applied; making the molten steel located at the upper portion to alloy molten steel by the stirring caused by the floating of the poured gas; and forming a surface layer composed of the alloy steel on the surface of the steel.
Furthermore, Japanese Unexamined Patent Application Publication No. 8-257692 discloses a method of manufacturing a cast piece having a uniform concentration of alloy element in the surface layer thereof by pouring molten steel having a predetermined composition, while forming a brake zone by applying a direct current magnetic field to a mold in the overall width thereof from a position a predetermined distance below a meniscus, using an immersion nozzle having the ejection holes of a nozzle above and blow the brake zone; and further continuously feeding alloy elements, which makes use of wires, to a molten steel pool above the brake zone and stirring the alloy element by the flow of the poured molten steel.
However, since the method disclosed in the Japanese Examined Patent Application Publication No. 3-20295 includes a very complicated process for separately refining the molten steel used in the surface layer of the cast piece and the molten steel used in the interior thereof, the method is liable to cause malfunction in production. Moreover, it is difficult to manufacture the cast piece stably in the method because it is necessary to perform very difficult control for independently supplying molten steel from respective tundishes in quantities according to the solidifying speeds thereof above and below the magnetic field zone. As a result, there is a problem that the yield of a product decreases.
As to this point, such strict control as described above is not required to the manufacturing method disclosed in Japanese Unexamined Patent Application Publication No. 7-51801. This is because that molten steel supplied from a tundish is one kind and the molten steel is supplied so as to maintain the molten metal in the mold to a predetermined level only below the magnetic field zone. Accordingly, the quantity of steel that is insufficient to a quantity of molten steel to be solidified above the magnetic field zone naturally flows from the lower portion of the magnetic field zone, and thus the strict control as described above is not necessary.
In this case, however, the molten steel slowly flows from the lower portion of the magnetic field zone to the upper zone thereof by the influence of the direct current magnetic filed. As a result, a problem arises in that an extreme difference of concentration between a portion to which a solute element is added and a portion apart from the above portion cannot be eliminated only by the stirring effect of bubbles.
Further, in the manufacturing method disclosed in Japanese Unexamined Patent Application Publication No. 8-257692, the same molten steel is supplied to upper and lower pools from the single nozzle having the ejection holes above and below the magnetic field zone. Thus, the method does not require a complicated process for separately preparing two types of molten steel.
In the method, however, the ratio of the quantities of molten steel to be supplied to the upper pool and the lower pool is controlled by adjusting the ratio of the inside diameters of the upper and lower ejection holes. Accordingly, when the ratio of the molten steel supplied to the lower pool is reduced even slightly, the boundary of the upper and lower molten steels having a different composition is offset from the magnetic field zone. Thus, the method is disadvantageous in that the alloy component in the upper pool drains to the lower pool and the yield of a product greatly decreases.
Conversely, the ratio of quantity of flow of molten steel to the upper pool decreases or a casting speed cannot help being reduced due to trouble, and the like in operation, molten steel containing a less quantity of alloy component flows in from the lower pool to the upper pool. At this time, since the molten steel that flows from the lower portion to the upper portion rises along both the ends of the mold in a width direction due to the influence of the stream of the molten steel from the lower ejection hole, a problem arises in that the alloy component decreases at both the ends of a cast piece and thus, the yield of a product greatly decreases, too.
An object of the present invention, which advantageously solves the above problems, is to propose an advantageous method of manufacturing a continuously-cast cast piece which not only permits the supply of molten steel to upper and lower pools to be easily controlled but also can simply and appropriately adjust the concentration of a solute element in the surface layer of the cast piece.
That is, the gist and the composition of the present invention is as shown below.
1. A method of manufacturing a continuously-cast cast piece by continuously casting molten steel in such a manner that a direct current magnetic field zone is applied to the cast piece over the entire width thereof in a direction across the thickness thereof at a position a predetermined distance below the molten metal level to casting direction in a continuously-casting mold and that the molten steel is poured using an immersion nozzle into a molten steel pool in or above the direct current magnetic field zone, the method comprising the steps of providing the immersion nozzle with ejection holes in at least upper and lower two stages; disposing a lower ejection hole(s) such that it satisfies the following formula (1); setting the supply speed Qxe2x80x2 of the molten steel supplied from upper ejection holes smaller than the speed Q consumed by solidification in the molten steel pool above the center of height of the direct current magnetic field zone; and adding a particular solute element to the molten steel in or above the direct current magnetic field zone, thereby adjusting the concentration of the solute element in the surface layer portion of the cast piece by increasing the concentration of the solute element as to the molten steel in the upper pool.
0 less than h less than (xc2xd)xc2x7wxc2x7tan xcex8xe2x80x83xe2x80x83(1)
where,
xcex8: downward angle of lower ejection hole(s) (xc2x0);
w: length of mold in width direction (m); and
h: distance from center of lower ejection hole to center of height of magnetic pole (m)
2. A method of manufacturing a continuously-cast cast piece according to above 1, wherein an immersion nozzle, which is designed such that the upper ejection holes satisfy the following formula (2), is used.
hxe2x80x2 greater than (xc2xd)xc2x7wxc2x7tan xcex8xe2x80x2xe2x80x83xe2x80x83(2)
where,
xcex8xe2x80x2: downward angle of upper ejection holes (xc2x0);
w: length of mold in width direction (m); and
hxe2x80x2: distance from center of upper ejection hole to center of height of magnetic pole (m)
3. A method of manufacturing a continuously-cast cast piece according to above 1 or 2, wherein an immersion nozzle, which is designed such that the upper ejection holes and the lower ejection hole(s) satisfy the following formulas (3) and (4), is used.
0 less than hxe2x89xa61.5Vxc2x7sin xcex8xe2x80x83xe2x80x83(3)
dxe2x89xa60.5xe2x80x83xe2x80x83(4)
where,
h: distance from center of lower ejection hole to center of height of magnetic pole (m);
V: average flow speed (m/s) of flow ejected from lower ejection hole(s) (m/s);
xcex8: downward angle of lower ejection hole(s) (xc2x0); and
d: distance from center of upper ejection hole to center of lower ejection hole (m)
4. A method of manufacturing a continuously-cast cast piece according to above 1, 2 or 3, wherein an immersion nozzle, which is designed such that the supply speed of the molten steel supplied from the upper ejection holes satisfies the following formula (5), is used.
0.3xc2x7Qxe2x89xa6Qxe2x80x2xe2x89xa60.9xc2x7Qxe2x80x83xe2x80x83(5)
where,
Qxe2x80x2: supply rate of molten steel supplied from upper ejection holes (ton/min)
Q: consumption rate of molten steel which solidifies in molten steel pool above center of height of magnetic pole (ton/min)